Bank Retreat Research Articles

Rainfall simulation to identify the storm-scale mechanisms of gully bank retreat

Abstract Gully erosion is one of the main causes of soil loss in drylands. Understanding the dominant mechanisms of erosion is important to achieve effective erosion control, thus in this study our main objective was to quantify the mechanisms involved in gully bank retreat as a result of three processes, falling of entire soil aggregates, transport of soil material by splash and by water running along gully banks (runoff), during rainfall events. The study was conducted in the sloping lands of the KwaZulu-Natal province, a region that is highly affected by gully erosion. Artificial rain was applied at 60 mm h −1 for 45 min at the vertical wall of a gully bank typical to the area. The splash material was collected by using a network of 0.045 m 2 buckets. The sediments in the running water were assessed by sampling the runoff collected from a microplot inserted within the base of the bank, and collecting the fallen aggregates after the rainfall simulation was complete. Results indicated that the overall erosion for the simulation was 721 g m −2 h −1 . Runoff erosion proved to be the dominant mechanism and amounted to 450 g m −2 h −1 , followed by splash and fall down of aggregates (about 170 g m −2 h −1 ). Gully bank retreat occurred at a rate of 0.55 mm h −1 and assuming that the soil bulk density is 1.3 g cm −3 , this corresponds to a retreat of 8.8 mm y −1 . Extrapolations to the watershed level, where about 500 m 2 of gully bank are observed per hectare, would lead to an erosion rate of 4.8 t ha −1 y −1 . These limited results based on a simulated storm show that the three main mechanisms (runoff, splash and fall down of aggregates) are responsible for the retreat of gully banks and that to mitigate gully erosion, appropriate measures are required to control all three mechanisms. Further research studies are needed to confirm and to scale up, both in time and space, as these data are obtained at one location and from a single artificial storm.

On the retreat of forested, cohesive riverbanks

Data from the Brandywine Creek in Pennsylvania and the South River in Virginia document spatial and temporal patterns describing the retreat of gently curving bends with forested riparian zones containing a mix of very large and smaller trees, cohesive banks, and slow rates of migration (∼ 0.1 m/yr). Field data include 2.5 years of bank profile surveys, measurements of tree morphology, and ground-based laser scanner (LiDAR) surveys repeated biannually. These banks retreat through a cycle of erosion on decadal timescales. Initially, small volumes (mean = 0.173 m 3 per bank erosion event) of soil are removed between large trees growing on the bank. Near trees, the rate of bank retreat is initially negligible, creating a scalloped bank morphology buttressed by large trees. As the trees are increasingly exposed to the flow, they become slowly undercut, gradually leaning into the channel and sliding down to the toe of the bank, eventually toppling into the river after a few years, which restarts the cycle. A magnitude–frequency analysis of monitoring data from the Brandywine Creek indicates that the effective bank erosion event only 0.11 m 3; thus, forested bank retreat is dominated by small soil failures. Large erosion events, many associated with trees toppling into the river, greatly increase the variance of time-averaged rates of bank erosion, such that almost 4 years of monitoring are needed to measure time-averaged rates of erosion to within an accuracy of 10%. Because of the limited spatial extent of our survey data and the absence of large discharges, these results are provisional. Improved process-based models to predict the retreat of forested riverbanks should account for the spatial structure of riparian forests and the interactions between the flow, the gradual removal of bank soils, and the ongoing efforts of the trees to remain attached to the bank.

A suburban inter-dike river reach of a large river: Modern morphological and sedimentary changes (the Bratislava reach of the Danube River, Slovakia)

The paper outlines contemporary geomorphic and sedimentary changes (1949–2007) in the suburban inter-dike reach of the Danube River in Bratislava underpinning requirements of its management. The studied river reach represents the right bank inter-dike area of a tectonically determined bend of the Danube River which is about 5 km long with a radius of 1.5 km. The landform and floodplain roughness classification schemes show variations in hydraulic and depositional conditions during flooding. Investigation of landform and floodplain roughness changes and the bank retreat rate is based on the multitemporal interpretation of aerial photographs. Sedimentological analysis of 10 borings and 20 pit exposures helped to delimit the morphostratigraphic units. The process and the rate of the vertical accretion of the modern floodplain were investigated by sedimentological and dendrochronological methods. The behaviour of the study reach is dependent on upstream dams in Austria. Dams produce changes in the suspended load regime and a steady riverbed erosion of 2 to 3.5 cm occurs upstream of the study river reach near the Slovak–Austrian border. The Čunovo dam downstream expediates upstream channel filling in spite of the ongoing channel gravel mining in the Danube channel. This has resulted in channel bed aggradation of about 1 m since 1992. The bank retreat of about 100 m during 1949–2007 was caused by the natural bank erosion processes as well as by channel straightening as a flood control measure for Bratislava. The bank shift resulted in the development of the new levee. The current overbank deposits differ from the older gravels, consisting of a fine-grained alluvium varying in thickness from 0.5 m to 1 m. The lithofacies of three flood deposits (March 2002, August 2002, and September 2007) were examined. Overbank sediments differentiate with distance from the bank and vertically. Suburban settings of the study reach and the flood control measures along with the external forcing (dams) induced changes in channel morphology as well as in floodplain roughness variability. This has resulted in changes in the present floodplain sedimentary conditions, channel forms and biodiversity. The landscape evolved through three distinct evolutionary phases.

Streambank dewatering for increased stability

AbstractStreambank erosion is often the dominant source of sediment leaving modified watersheds. Mass failure of high, steep banks is one of the most serious forms of streambank erosion. The risk of a given bank experiencing mass failure is a function of bank height, angle, and soil strength, which is governed by soil moisture. Two methods for bank dewatering were tested in adjacent sections of streambank bordering a deeply incised channel in northern Mississippi: a low‐cost pump system and subsurface horizontal drains. Pore water pressures (both positive and negative pressures, or matric suction) were continuously monitored for 2 years at the pumped site, at an adjacent untreated control section, and for 1 year at the site stabilized with horizontal drains. Resulting data were used to calculate a time series of the factor of safety using a computer model. Over the course of two wet seasons, average bank retreats for the control and pumped plots were 0·43 and 0·21 m, respectively. More limited monitoring revealed that the site with passive drains retreated about 0·23 m. At the pumped site pore water pressure was 3–4 kPa lower than at the control site during the most critical periods. Accordingly, computed factors of safety were above the failure threshold at the pumped site, but fell below unity at the control site on 11 occasions over the period of observation. Similarly, the drained site displayed generally lower pore water pressure and higher safety factors except for two events when drains were evidently overwhelmed with the volume of local surface and subsurface flows. These results suggest, but do not prove, that bank dewatering promoted lower rates of bank retreat and higher levels of stability since the three sites had slight differences in soils, geometry and boundary conditions. Initial cost of the dewatering treatments were significantly less than orthodox bank stabilization measures, but operation and maintenance requirements may be greater. Published in 2009 by John Wiley & Sons, Ltd.

Monitoring and numerical modelling of riverbank erosion processes: a case study along the Cecina River (central Italy)

AbstractRiverbank retreat along a bend of the Cecina River, Tuscany (central Italy) was monitored across a near annual cycle (autumn 2003 to summer 2004) with the aim of better understanding the factors influencing bank changes and processes at a seasonal scale. Seven flow events occurred during the period of investigation, with the largest having an estimated return period of about 1·5 years. Bank simulations were performed by linking hydrodynamic, fluvial erosion, groundwater flow and bank stability models, for the seven flow events, which are representative of the typical range of hydrographs that normally occur during an annual cycle. The simulations allowed identification of (i) the time of onset and cessation of mass failure and fluvial erosion episodes, (ii) the contributions to total bank retreat made by specific fluvial erosion and mass‐wasting processes, and (iii) the causes of retreat. The results show that the occurrence of bank erosion processes (fluvial erosion, slide failure, cantilever failure) and their relative dominance differ significantly for each event, depending on seasonal hydrological conditions and initial bank geometry. Due to the specific planimetric configuration of the study bend, which steers the core of high velocity fluid away from the bank at higher flow discharges, fluvial erosion tends to occur during particular phases of the hydrograph. As a result fluvial erosion is ineffective at higher peak discharges, and depends more on the duration of more moderate discharges. Slide failures appear to be closely related to the magnitude of peak river stages, typically occurring in close proximity to the peak phase (preferentially during the falling limb, but in some cases even before the peak), while cantilever failures more typically occur in the late phase of the flow hydrograph, when they may be induced by the cumulative effects of any fluvial erosion. Copyright © 2008 John Wiley & Sons, Ltd.

Identifying the effects of parameter uncertainty on the reliability of riverbank stability modelling

Bank retreat is a key process in fluvial dynamics affecting a wide range of physical, ecological and socioeconomic issues in the fluvial environment. To predict the undesirable effects of bank retreat and to inform effective measures to prevent it, a wide range of bank stability models have been presented in the literature. These models typically express bank stability by defining a factor of safety as the ratio of driving and resisting forces acting on the incipient failure block. These forces are affected by a range of controlling factors that include such aspects as the bank profile (bank height and angle), the geotechnical properties of the bank materials, as well as the hydrological status of the riverbanks. In this paper we evaluate the extent to which uncertainties in the parameterization of these controlling factors feed through to influence the reliability of the resulting bank stability estimate. This is achieved by employing a simple model of riverbank stability with respect to planar failure (which is the most common type of bank stability model) in a series of sensitivity tests and Monte Carlo analyses to identify, for each model parameter, the range of values that induce significant changes in the simulated factor of safety. These identified parameter value ranges are compared to empirically derived parameter uncertainties to determine whether they are likely to confound the reliability of the resulting bank stability calculations. Our results show that parameter uncertainties are typically high enough that the likelihood of generating unreliable predictions is typically very high (> ∼ 80% for predictions requiring a precision of < ± 15%). Because parameter uncertainties are derived primarily from the natural variability of the parameters, rather than measurement errors, much more careful attention should be paid to field sampling strategies, such that the parameter uncertainties and consequent prediction unreliabilities can be quantified more robustly.

Quantifying bank erosion on the South River from 1937 to 2005, and its importance in assessing Hg contamination

Bank sediments along a 40 km reach of the South River, downstream of Waynesboro, VA, store mercury from historical contamination as a result of textile manufacturing. Knowledge of the rate at which contaminated sediment is released to the stream channel through bank erosion is required to implement restoration programs designed, for example, to minimize its ecological impact and to reduce risk to human health. Digitized stream channel boundaries based on visual interpretations of georeferenced aerial imagery from 1937 and 2005 were compared to calculate a minimum estimate of the total area of bank sediment eroded between Waynesboro and Port Republic, Virginia. Estimates of riverbank height were extracted from aerial LIDAR data, allowing areal estimates of bank retreat to be converted to volumes. Nominal annual rates of bank retreat, averaged over the 68-year period, for several example locales along the study reach are very low, ranging from 3 to 15 cm per year. Bank erosion occurs at the outside banks of bends, through the development of islands, where deposition on confluence bars pushes the main flow into the opposite bank, and in small areas along the channel that are difficult to classify or explain. A minimum estimate of the total volume eroded for the study reach is approximately 161,000 m 3; the corresponding annual mass of mercury supplied to the channel by bank erosion is 109.6 kg/year. Our work demonstrates that a careful analysis of aerial imagery and LIDAR data can provide detailed, spatially explicit estimates of mercury loading from bank erosion, even when rates of riverbank erosion are unusually low.

Numerical simulation of hydrodynamics and bank erosion in a river bend

We present an integrated analysis of bank erosion in a high‐curvature bend of the gravel bed Cecina River (central Italy). Our analysis combines a model of fluvial bank erosion with groundwater flow and bank stability analyses to account for the influence of hydraulic erosion on mass failure processes, the key novel aspect being that the fluvial erosion model is parameterized using outputs from detailed hydrodynamic simulations. The results identify two mechanisms that explain how most bank retreat usually occurs after, rather than during, flood peaks. First, in the high curvature bend investigated here the maximum flow velocity core migrates away from the outer bank as flow discharge increases, reducing sidewall boundary shear stress and fluvial erosion at peak flow stages. Second, bank failure episodes are triggered by combinations of pore water and hydrostatic confining pressures induced in the period between the drawdown and rising phases of multipeaked flow events.

Destabilization of streambanks by removal of invasive species in Canyon de Chelly National Monument, Arizona

As part of a study to investigate the causes of channel narrowing and incision in Canyon de Chelly National Monument, the effects of Tamarisk and Russian-olive on streambank stability were investigated. In this study, root tensile strengths and distributions in streambanks were measured and used in combination with a root-reinforcement model, RipRoot, to estimate the additional cohesion provided to layers of each streambank. The additional cohesion provided by the roots in each 0.1-m layer ranged from 0 to 6.9 kPa for Tamarisk and from 0 to 14.2 kPa for Russian-olive. Average root-reinforcement values over the entire bank profile were 2.5 and 3.2 kPa for Tamarisk and Russian-olive, respectively. The implications of vegetation removal on bank stability and failure frequency were evaluated in two incised reaches by modeling bank-toe erosion and bank stability with and without vegetation. The effects of a series of 1.0- and 1.5-m-deep flows on bank-toe erosion, pore-pressure distributions, and bank stability were evaluated first. In addition, bank stability model runs were conducted using iterative modeling of toe erosion and bank stability using a discretized flow and groundwater record for one year. Results showed that the effects of root-reinforcement provided by Tamarisk and Russian-olive have a significant impact on bank-stability and bank-failure frequency. Because the bank materials are dominated by sands, cohesion provided by roots is significant to bank stability, providing an average 2.8 kPa of cohesion to otherwise cohesionless bank materials. Bank retreat rates at one site following vegetation removal have approximately doubled when compared to the control reach (from an approximate rate of 0.7–0.8 m/y between 2003 and 2006 to 1.85 m/y during the year modeled). Vegetation removal along the entire riparian corridor in Canyon de Chelly may lead to the introduction of significantly more sediment to the system through bank widening processes, although it is not known whether this change alone would be sufficient to cause a shift in channel morphology to the wide-braided channels that were once characteristic of this canyon.

The effects of variability in bank material properties on riverbank stability: Goodwin Creek, Mississippi

Bank retreat is an important area of research within fluvial geomorphology and is a land management problem of global significance. The Yazoo River Basin in Mississippi is one example of a system which is experiencing excessive erosion and bank instability. The properties of bank materials are important in controlling the stability of stream banks and past studies have found that these properties are often variable spatially. Through an investigation of bank material properties on a stretch of Goodwin Creek in the Yazoo Basin, Mississippi, this study focuses on: i) how and why effective bank material properties vary through different scales; ii) how this variation impacts on the outputs from a bank stability model; and iii) how best to appropriately represent this variability within a bank stability model. The study demonstrates the importance that the variability of effective bank material properties has on bank stability: at both the micro-scale within a site, and at the meso-scale between sites in a reach. This variability was shown to have important implications for the usage of the Bank Stability and Toe Erosion Model (BSTEM), a deterministic bank stability model that currently uses a single value to describe each bank material property. As a result, a probabilistic representation of effective bank material strength parameters is recommended as a potential solution for any bank stability model that wishes to account for the important influence of the inherent variability of soil properties.

Coupled simulations of fluvial erosion and mass wasting for cohesive river banks

The erosion of sediment from riverbanks affects a range of physical and ecological issues. Bank retreat often involves combinations of fluvial erosion and mass wasting, and in recent years, bank retreat models have been developed that combine hydraulic erosion and limit equilibrium stability models. In related work, finite element seepage analyses have also been used to account for the influence of pore water pressure in controlling the onset of mass wasting. This paper builds on these previous studies by developing a simulation modeling approach in which the hydraulic erosion, finite element seepage, and limit equilibrium stability models are, for the first time, fully coupled. Application of the model is demonstrated by undertaking simulations of a single flow event at a single study site for scenarios where (1) there is no fluvial erosion and the bank geometry profile remains constant throughout, (2) there is no fluvial erosion but the bank profile is deformed by simulated mass wasting, and (3) the bank profile is allowed to freely deform in response to both simulated fluvial erosion and mass wasting. The results are limited in scope to the specific conditions encountered at the study site, but they nevertheless demonstrate the significant role that fluvial erosion plays in steepening the bank profile or creating overhangs, thereby triggering mass wasting. However, feedbacks between the various processes also lead to unexpected outcomes. Specifically, fluvial erosion also affects bank stability indirectly, as deformation of the bank profile alters the hydraulic gradients driving infiltration into the bank, thereby modulating the evolution of the pore water pressure field. Consequently, the frequency, magnitude, and mode of bank erosion events in the fully coupled scenario differ from the two scenarios in which not all the relevant bank process interactions are included.

Advances in the continuous monitoring of erosion and deposition dynamics: Developments and applications of the new PEEP-3T system

In most episodic erosion and deposition systems, knowledge of the timing of geomorphological change, in relation to fluctuations in the driving forces, is crucial to strong erosion process inference, and model building, validation and development. A challenge for geomorphology, however, is that few studies have focused on geomorphological event structure (timing, magnitude, frequency and duration of individual erosion and deposition events), in relation to applied stresses, because of the absence of key monitoring methodologies. This paper therefore (a) presents full details of a new erosion and deposition measurement system — PEEP-3T — developed from the Photo-Electronic Erosion Pin sensor in five key areas, including the addition of nocturnal monitoring through the integration of the Thermal Consonance Timing (TCT) concept, to produce a continuous sensing system; (b) presents novel high-resolution datasets from the redesigned PEEP-3T system for river bank system of the Rivers Nidd and Wharfe, northern England, UK; and (c) comments on their potential for wider application throughout geomorphology to address these key measurement challenges. Relative to manual methods of erosion and deposition quantification, continuous PEEP-3T methodologies increase the temporal resolution of erosion/deposition event detection by more than three orders of magnitude (better than 1-second resolution if required), and this facility can significantly enhance process inference. Results show that river banks are highly dynamic thermally and respond quickly to radiation inputs. Data on bank retreat timing, fixed with PEEP-3T TCT evidence, confirmed that they were significantly delayed up to 55 h after flood peaks. One event occurred 13 h after emergence from the flow. This suggests that mass failure processes rather than fluid entrainment dominated the system. It is also shown how, by integrating turbidity instrumentation with TCT ideas, linkages between sediment supply and sediment flux can be forged at event timescales, and a lack of sediment exhaustion was evident here. Five challenges for wider geomorphological process investigation are discussed. This event-based dynamics approach, based on continuous monitoring methodologies, appears to have considerable wider potential for stronger process inference and model testing and validation in many areas of geomorphology.

Experimental study of rill bank collapse

AbstractRill bank collapse is an important component in the adjustment of channel morphology to changes in discharge and sediment flux. Sediment inputs from bank collapse cause abrupt changes in flow resistance, flow patterns and downstream sediment concentrations. Generally, bank retreat involves gradual lateral erosion, caused by flow shear stress, and sudden bank collapse, triggered by complex interactions between channel flow and bank and soil water conditions. Collapse occurs when bank height exceeds the critical height where gravitational forces overcome soil shear strength. An experimental study examined conditions for collapse in eroding rill channels. Experiments with and without a deep water table were carried out on a meandering rill channel in a loamy sand and sandy loam in a laboratory flume under simulated rainfall and controlled runon. Different discharges were used to initiate knickpoint and rill incision. Soil water dynamics were monitored using microstandpipes, tensiometers and time domain reflectometer probes (TDR probes). Bank collapse occurred with newly developed or rising pre‐existing water tables near rill banks, associated with knickpoint migration. Knickpoint scour increased effective bank height, caused positive pore water pressure in the bank toe and reduced negative pore pressures in the unsaturated zone to near zero. Matric tension in unsaturated parts of the bank and a surface seal on the ‘interrill’ zone behind the bank enhanced stability, while increased effective bank height and positive pore water pressure at the bank toe caused instability. With soil water contents &gt;35 per cent (sandy loam) and &gt;23 per cent (loamy sand), critical bank heights were 0·11–0·12 m and 0·06–0·07 m, respectively. Bank toe undercutting at the outside of the rill bends also triggered instability. Bank displacement was quite different on the two soils. On the loamy sand, the failed block slid to the channel bed, revealing only the upper half of the failure plane, while on the sandy loam the failed block toppled forwards, exposing the failure plane for the complete bank height. This study has shown that it is possible to predict location, frequency and magnitude of the rill bank collapse, providing a basis for incorporation into predictive models for hillslope soil loss or rill network development. Copyright © 2006 John Wiley &amp; Sons, Ltd.

Defining the moment of erosion: the principle of thermal consonance timing

AbstractGeomorphological process research demands quantitative information on erosion and deposition event timing and magnitude, in relation to fluctuations in the suspected driving forces. This paper establishes a new measurement principle – thermal consonance timing (TCT) – which delivers clearer, more continuous and quantitative information on erosion and deposition event magnitude, timing and frequency, to assist understanding of the controlling mechanisms. TCT is based on monitoring the switch from characteristically strong temperature gradients in sediment, to weaker gradients in air or water, which reveals the moment of erosion. The paper (1) derives the TCT principle from soil micrometeorological theory; (2) illustrates initial concept operationalization for field and laboratory use; (3) presents experimental data for simple soil erosion simulations; and (4) discusses initial application of TCT and perifluvial micrometeorology principles in the delivery of timing solutions for two bank erosion events on the River Wharfe, UK, in relation to the hydrograph.River bank thermal regimes respond, as soil temperature and energy balance theory predicts, with strong horizontal thermal gradients (often &gt;1 K cm−1 over 6·8 cm). TCT fixed the timing of two erosion events, the first during inundation, the second 19 h after the discharge peak and 13 h after re‐emergence from the flow. This provides rare confirmation of delayed bank retreat, quantifies the time‐lag involved, and suggests mass failure processes rather than fluid entrainment. Erosion events can be virtually instantaneous, implying ‘catastrophic retreat’ rather than ‘progressive entrainment’. Considerable potential exists to employ TCT approaches for: validating process models in several geomorphological contexts; assisting process identification and improving discrimination of competing hypotheses of process dominance through high‐resolution, simultaneous analysis of erosion and deposition events and driving forces; defining shifting erodibility and erosion thresholds; refining dynamic linkages in event‐based sediment budget investigations; and deriving closer approximations to ‘true’ erosion and deposition rates, especially in self‐concealing scour‐and‐fill systems. Copyright © 2005 John Wiley &amp; Sons, Ltd.

Wood storage in a wide mountain river: case study of the Czarny Dunajec, Polish Carpathians

AbstractStorage of large woody debris in the wide, mountain, Czarny Dunajec River, southern Poland, was investigated following two floods of June and July 2001 with a seven‐year frequency. Within a reach, to which wood was delivered only by bank erosion and transport from upstream, wood quantities were estimated for eighty‐nine, 100 m long, channel segments grouped into nine sections of similar morphology. Results from regression analysis indicated the quantity of stored wood to be directly related to the length of eroded, wooded banks and river width, and inversely related to unit stream power at the flood peak. The largest quantities of wood (up to 33 t ha−1) were stored in wide, multi‐thread river sections. Here, the relatively low transporting ability of the river facilitated deposition of transported wood while a considerable length of eroded channel and island banks resulted in a large number of trees delivered from the local riparian forest. In these sections, a few morphological and ecological situations led to the accumulation of especially large quantities of wood within a small river area. Very low amounts of wood were stored in narrow, single‐thread sections of regulated or bedrock channel. High stream power facilitated transport of wood through these sections while the high strength of the banks and low channel sinuosity prevented bank retreat and delivery of trees to the channel.Considerable differences in the character of deposited wood existed between wide, multi‐thread channel sections located at different distances below a narrow, 7 km long, channellized reach of the river. Wood deposited close to the downstream end of the channellized reach was highly disintegrated and structured into jams, whereas further downstream well preserved shrubs and trees prevailed. This apparently reflects differences in the distance of wood transport and shows that in a mountain river wider than the height of trees growing on its banks, wood can be transported long distances along relatively narrow, single‐thread reaches but is preferentially deposited in wide, multi‐thread reaches. Copyright © 2005 John Wiley &amp; Sons, Ltd.

Formation of oriented thaw lakes by thaw slumping

In the classic model for oriented thaw lakes, sublittoral shelves form by wind‐driven circulation near shorelines oriented perpendicular to the wind, protecting the adjacent banks from thaw and wave cut erosion. Here I propose an alternative model based on thaw slumping and test the model predictions against observations in northern Alaska. Thermal modeling illustrates that bank height controls the rate of thaw slumping because summertime thaw penetrates only decimeters into a tall bank but as much as tens of meters into a short bank. This effect also leads to oriented lakes because of a nonlinear relationship between bank height and bank retreat rate. Bank material texture also controls the rate of thaw slumping because fine‐grained sediments drain slowly and maintain higher pore pressures than coarse‐grained sediments, resulting in lower critical angles for slumping. To test the thaw slumping model quantitatively, I constructed a process‐based numerical model that includes thaw slumping, lacustrine sediment dispersal, and thaw‐driven lake floor subsidence. The model predicts that lake orientations and aspect ratios are controlled by topographic aspect and slope, not by wind direction and intensity. The thaw slumping model further predicts inverse correlations between lake area and bank height and between lake area and bank material texture. An analysis of oriented thaw lakes in northern Alaska shows that systematically smaller, deeper lakes form in coarse‐grained eolian sediments compared with those formed in fine‐grained fluvial marine sediments. This pattern strongly supports the thaw slumping model.

Fluvial thermal erosion investigations along a rapidly eroding river bank: application to the Lena River (central Siberia)

AbstractIn Central Yakutia, frozen river banks are affected by a combination of thermal and mechanical erosion. Exceptional bank retreat of up to 40 m per year is observed. This results from ground thawing produced by heat transfer from the flow of water through the frozen ground, followed by mechanical transport of the thawed sediments. A one‐dimensional model is proposed to estimate the thermal erosion efficiency. A test of this model is a comparison of results obtained from experiments carried out in a cold room. A hydraulic channel allows measurements of the thaw front propagation, as well as the thermal erosion rate, in simulated ground ice that is subjected to warm water flow. Various laboratory simulations demonstrate the validity of the mathematical model for the range of laboratory conditions. A hierarchy of parameters (Reynolds number, water and ground ice temperatures) is proposed to explain the present efficiency of thermal erosion along the Siberian rivers. From the characteristics of the Lena River (geometry, temperature and discharge) during the flood season, the erosion of banks with different ice content predicted by the model is in agreement with field observations. Copyright © 2003 John Wiley &amp; Sons, Ltd.

Three‐dimensional flow structure and channel change in an asymmetrical compound meander loop, Embarras River, Illinois

AbstractThe planform dynamics of meandering rivers produce a complex array of meander forms, including elongated meander loops. Thus far, few studies have examined in detail the flow structure within meander loops and the relation of flow structure to patterns of planform change. This field‐based investigation examines relations between three‐dimensional fluid motion and channel change within an elongated, asymmetrical meander loop containing multiple pool–riffle structures. The downstream velocity field is characterized by a high‐velocity core that shifts slightly outward as flow moves through individual lobes of the loop. For some of the measured flows this core becomes submerged below the water surface downstream of the lobe apexes. Vectors of cross‐stream/vertical velocities indicate that skew‐induced helical motion develops within the pools near lobe apexes and decays over riffles where channel curvature is less pronounced. Maximum rates of bank retreat generally occur near lobe apexes where impingement of the flow on the outer channel bank is greatest. However, maximum rates and loci of bank retreat differ for upstream and downstream lobes of the loop, leading to increasing asymmetry of loop geometry over time—a finding consistent with experimental investigations of loop evolution. Copyright © 2003 John Wiley &amp; Sons, Ltd.

Estimating Boat-Wake-Induced Levee Erosion using Sediment Suspension Measurements

The subaqueous portion of a levee bank in the Sacramento–San Joaquin River Delta of central California was instrumented to quantify the impact of boat-generated waves. Typical erosion rates associated with recreational craft are too small for direct measurement of bank retreat on a per-boat-passage basis; therefore, two independent analytical methods of estimating linear erosion were developed based on colocated suspended sediment concentration and velocity time series. The algorithms were tested using data measured during a field experiment in which a 7.5 m boat was driven past the site over a range of speeds to generate waves of varying size. A cross-shore array of electromagnetic current meters and optical back-scatterance sensors measured the character of boat-generated waves and the resultant sediment suspension. In near-bank, shallow-water (d<0.5m) locations, sediment suspension was closely correlated with the primary boat-wake waves (Hmax<0.21m), indicating that maximum near-bottom orbital velocities were sufficient to erode the fine-grained (mud-silt) bottom materials. Suspension events were short lived (order of 1–5 min), despite very long particle settling times (order of hours), because river currents swept the suspension plumes downstream. This implies negligible sedimentation and resuspension locally. Both algorithms produced strikingly similar erosion estimates, and these values (0.01–0.22 mm/boat passage) compare favorably with direct measurements of cumulative bank erosion in response to multiple, sequential boat passages. Field conditions for which the algorithms are appropriately applied are discussed.

Streambank and vegetation response to simulated cattle grazing

Simulated grazing techniques were used to investigate livestock impacts on structural and vegetation characteristics of streambanks in central Idaho, USA. The treatments, continued over two years, consisted of no grazing, simulated moderate early summer grazing, simulated moderate mid-summer grazing, and simulated heavy season-long grazing. The moderate treatments depressed the streambank surface about 3 cm, while the heavy season-long treatment resulted in an 11.5-cm depression. There were no differences between the no-grazing and moderate-grazing treatments for change in stream width, bank angle, bank retreat, or root biomass. The heavy season-long treatment, however, produced significant changes in these variables. The amount of foliage biomass (i.e., kg ha−1) removed by treatment was similar between the two years of study for the moderate treatments. The foliage removed from the heavy season-long treatment plots greatly decreased in the second year as plant growth decreased. Ten months after the last treatment application, the average spring foliage growth was 20–43% lower on the moderate treatment plots and 51–87% lower on the heavy season-long treatment plots than on the untreated control plots.